1. Field of the Invention
This invention relates to a process for producing a toner for developing electrostatic latent images which is used in electrophotography.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, etc. are known as electrophotography, which is commonly a process in which, using a photoconductive material, copies are obtained by forming an electrostatic latent image on a photosensitive member by various means, subsequently developing the latent image by the use of a toner to form a toner image, transferring the toner image to a recording medium such as paper as occasion calls, and thereafter fixing the toner image by the action of heat and/or pressure or solvent vapor. As methods for developing the electrostatic latent image by the use of toners or methods for fixing the toner image, a variety of methods have been proposed, and methods suited for the respective image forming processes are employed. Further in recent years, electrophotography is sought to achieve higher-speed copying and higher image quality.
Toners used for such purpose have been commonly produced by melt-kneading colorants comprising dyes or pigments, into thermoplastic resins to effect uniform dispersion, followed by pulverization using a fine grinding mill, and the pulverized product is classified using a classifier to produce toners having the desired particle diameters.
Reasonably good toners can be produced by such a production method, but there is a certain limit, i.e., a limit to the range in which toner materials are selected. For example, colorant-dispersed resin compositions must be brittle enough to be pulverizable with ease by means of a fine grinding mill economically available. However, such colorant-dispersed resin compositions made brittle in order to meet these requirements tend to result in a broad particle size distribution of the particles formed when actually pulverized at a high speed, causing especially a problem that fine particles tend to be produced in a relatively large proportion. Moreover, such highly brittle materials tend to be further finely pulverized or powdered when used for the development in copying machines or the like. Also, in this method, it is difficult to uniformly disperse solid fine particles of colorants and the like in the resin, and toners may cause an increase in fog, a decrease in image density and a lowering of color mixing properties or transparency of toners, depending on the degree of such dispersion. Accordingly, care must well be taken when colorants are dispersed. Also, colorants may come bare to rupture sections of pulverized particles, and may cause fluctuations in developing performance of toners.
Meanwhile, in order to overcome the problems of the toners produced by such pulverization, toners produced by suspension polymerization are proposed as disclosed in Japanese Patent Publications No. 36-10231, No. 43-10799 and No. 51-14895. In the production of toners by suspension polymerization, a polymerizable monomer, a colorant and a polymerization initiator, and also optionally a cross-linking agent, a charge control agent and other additives are uniformly dissolved or dispersed to form a polymerizable monomer composition. Thereafter, this polymerizable monomer composition is dispersed in a continuous phase containing a dispersion stabilizer, e.g., in an aqueous medium, by means of a suitable agitator, and is subjected simultaneously to polymerization reaction to obtain toner particles having the desired particle diameters.
Since this method has no step of pulverization, the toner particles are not required to be brittle, and hence soft materials can be used as the resin. Also, colorants may hardly come bare to the surfaces of toner particles, and hence the toners can have a uniform triboelectric charging performance. This method has such advantages. Since also the toners obtained have a relatively sharp particle size distribution, the step of classification can be omitted, or, even when classified, toners can be obtained in a high yield. The method also has the advantage that, since a low-softening substance as a release agent can be encapsulated in toner particles in a large quantity, the toners obtained have a superior anti-offset properties.
As examples where polymerization toners are used, it is proposed in Japanese Patent Application Laid-open No. 5-19662 to use secondary toner particles obtained by fusing primary polymerization particles; in Japanese Patent Application Laid-open No. 4-296766 to use a polymerization toner that transmits the light to which the photosensitive member is exposed; and in Japanese Patent Application Laid-open No. 5-188637 to use a toner specified in its volume-average particle diameter, number-average particle diameter, charge quantity of toner, projected-image area ratio of toner and BET specific surface area of toner. These, however, have not been well satisfactory.
Toners like the polymerization toners, having spherical particles and smooth particle surfaces, have small points of contact with charging members because of such features, and may cause problems of a lowering of charging performance and, in an environment of high humidity, a great lowering of charging performance. In order to solve these problems, it has been required for such toners to be more improved in charging performance than conventional toners.
Conventionally, in order to make a toner hold electric charges, it is possible to utilize the triboelectric chargeability of a resin which is a component of the toner. In this method, however, the toner has so low a charging performance that images obtained by development tend to fog and be unsharp. Accordingly, in order to impart the desired triboelectric charging performance to the toner, it is common to add a dye or pigment capable of imparting chargeability and further add a charge control agent.
Toners containing a charge control agent tend to contaminate toner carrying members such as developing sleeves, and hence the quantity of triboelectricity decreases with an increase in the number of sheets during copying to tend to cause a decrease in image density. Some charge control agents also have an insufficient quantity of triboelectricity and tend to be affected by temperature and humidity, tending to cause variations of image density which occur with environmental variations. Some charge control agents also have a poor dispersibility in resins, and hence tones making use of such charge control agents tend to have a non-uniform quantity of triboelectricity between toner particles, tending to cause fog. Some charge control agents still also have so poor a storage stability that the quantity of triboelectricity tends to decrease during long-term storage.
In polymerization toners, when materials are granulated in an aqueous medium, charge control agents tend to affect the system adversely. For example, problems may arise such that pigments are not dispersed well or that charge control agents dissolve partly to cause fine particles. In particular, formation of very fine particles such as emulsion particles causes contamination of charging members and great damage of toner performance, e.g., inhibition of charging performance of the toners.
Nowadays, charge control agents known in the present technical field include, as negative charge control agents, metal complexes of monoazo dyes, metal complexes of salicylic acid, naphthoic acid or dicarboxylic acids, copper phthalocyanine pigments, and resins containing acid components. Known as positive charge control agents are nigrosine dyes, azine dyes, triphenylmethane dyes, quaternary ammonium salts, and resins having a quaternary ammonium salt in the side chain.
Of these, various metal complexes are proposed as negative charge control agents, as disclosed in Japanese Patent Publications No. 43-17955 and No. 55-42752, Japanese Patent Application Laid-open No. 61-155464, Japanese Patent Publication No. 63-1994 and so froth. These certainly exhibit good chargeability, but, according to studies made by the present inventors, it can not be said that they are satisfactory when used in polymerization toners. Thus, it has been sought to make an improvement.